Apple is easier than Google in a lot of ways, but more complex in others.
Rob
Or just publish it to the App Store and then you can install it on anything. It’s not like you have to promote it to the general public. I’ll bet there are literally tens of thousands of apps on there right now that NOBODY except the developer and his mom have ever seen.
And there are tons of apps there right now that are USELESS without an accompanying account, and you can’t even make an account in the app. So you never have to allow the general public to be able to use the app at all.
For example, search for SAP Concur on the app store – it says right in the description it’s only for existing Concur users.
So just make an app, publish it, and don’t worry about how to do something weird. 
Jay
You’re right Jay. Adrian said exactly the same yesterday, and in all honesty I’d just taken the clients request for ad-hoc and rolled with it, not considering at all the possibility of just locking it up with some kind of invite code and using the store for distribution anyway. I’ve proposed this to the client now. 